It is frequently necessary to measure the concentrations of individual amino acids in either clear or turbid solutions in aqueous liquids, including physiological fluids such as blood serums and spinal fluids. This has proved difficult or even impossible to achieve with existing technology. However, two of the present inventors have achieved success in this regard by the use of membrane electrodes utilizing living bacterial cells, as described in their publication Science, vol. 199, pp. 440-41 (1978), authors Rechnitz, Riechel, Kobos and Meyerhoff.